A short wavelength light source apparatus

Abstract: A short wavelength light source apparatus according to the present invention includes: a semiconductor laser (101) including an active layer for emitting laser light; and an optical wavelength converting element (109) for receiving at least a portion of the laser light emitted from the semiconductor laser and for generating short wavelength light having a wavelength shorter than a wavelength of the laser light, wherein the short wavelength light source apparatus further includes an optical element (103) for selectively reflecting a portion of the laser light that belongs to a predetermined wavelength band and for feeding back the reflected light to the active layer of the semiconductor laser (101), the optical element (103) being disposed between the semiconductor laser (101) and the optical wavelength converting element (109).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a light source apparatus for use in a high-density optical disk system and the like, and particularly to a short wavelength light source apparatus incorporating a semiconductor laser as a pump light source. 2. Description of the Related Art: In the field of high density recording using optical disks and image processing, there has been a need for light source apparatuses which incorporate semiconductor lasers as pump light sources and which provide green light or blue light by converting light emitted from the semiconductor lasers into light having different wavelengths. Such a light source apparatus is required to satisfy the following stipulations. 1) The output light has a Gaussian light intensity distribution in the lateral direction, and light can be focused at the diffraction limit. 2) The optical power of the output light is on the order of a few mW (milliwatts), and the operation is stable both in terms of the generated frequency and as a function of time. In order to obtain a short wavelength light source apparatus having an output power of a few mW or more with a semiconductor laser incorporated as a pump light source, a method for directly converting the wavelength of semiconductor laser light is regarded as promising. The method typically uses, as an optical wavelength converting element, a polarization inversion type optical waveguide element of a quasi-phase matching (hereinafter referred to as "QPM") type using LiTaO 3 , LiNbO 3 , and KTiOPO 4 as a substrate (Yamamoto et al.; Optics Letters Vol.16, No.15, 1156 (1991)), a polarization inversion type bulk element, and/or a phase matching type non-linear optical crystal having a large non-linear optical constant, e.g. KNbO 3 (KN). Also regarded as promising are intracavity type short wavelength light source apparatuses incorporating cavities internalizing laser media. In such a light source apparatus, a laser medium receives laser light generated by a semiconductor laser (pump light source) so as to effect laser oscillation, and generates laser light having a longer wavelength than that of the laser light from the semiconductor laser. The generated laser light is converted into short-wavelength light (harmonics) by an optical wavelength converting element inserted in the interior of a resonator. First, a direct wavelength conversion method using a polarization inversion type waveguide element will be described. Currently, in devices incorporating polarization inversion type waveguide element of the QPM type, blue light having an optical output power of 1.1 mW is obtained under the condition that light emitted from a semiconductor laser and entering the polarization inversion type waveguide has an intensity of 35 mW. However, while the QPM polarization inversion type waveguide element having an allowable range of wavelength variation of only 0.2 nm, the light oscillated by the semiconductor laser has a fluctuation as large as 0.2 nm/°C against temperature changes. Moreover, since there is a mode hopping of about 1 nm due to light returning to the semiconductor laser, the output of the QPM polarization inversion type waveguide element can be stable for a time period of only several seconds. Phase matching type non-linear optical crystals and QPM polarization inversion type bulk elements similarly have a relatively narrow range of allowable wavelength variation with respect to phase matching. This increases the importance of stabilization of the oscillation wavelength of a semiconductor laser used as a pump light source. With a view to stabilizing the oscillation wavelength of a light generating apparatus incorporating a semiconductor laser as a pump light source, there has been proposed a short wavelength light source apparatus in which a grating is provided by the end of a semiconductor laser through which laser light goes out. The general configuration of this conventional short wavelength light source apparatus will be described with reference to Figure 13 . In Figure 13 , reference numeral D01 denotes a 50 mW-class AlGaAs semiconductor laser for the 0.83 µm band; D02 denotes a collimating lens; D03 denotes a λ/2 plate; D04 denotes a focusing lens having a numerical aperture (hereinafter referred to as "N.A.") of 0.6; and D05 denotes a grating disposed at an angle of  with the optical axis of the semiconductor laser D01 . The grating D05 has a linear shape. A high reflectance coating is formed on an end face D06 of the semiconductor laser D01 . The grating D05 has a wavelength dispersing effect, and therefore is capable of locking the oscillation wavelength of the semiconductor laser D01 by feeding back light of a certain wavelength to the semiconductor laser D01 as a first-order diffracted light. Laser light (wavelength: 830 nm) reflected from the grating D05 is incident to the λ/2 plate D03 , which rotates the polarizing direction of the laser light, and is lead through a focusing lens D04 so as to be focused on an end face D07 of the polarization inversion type waveguide element D08 . Light propagated through the polarization inversion type waveguide element D08 having polarization-inverted layers (period: 3.7 µm) is converted into light with a wavelength of 415 nm so as to go out through an end face D09 . An antireflection (AR) coating which does not reflect the fundamental wave is provided on each of the end faces D07 and D09 . The polarization inversion type waveguide element D08 is formed on an LiTaO 3 substrate. Next, a solid laser incorporating a semiconductor laser as a pump light source will be described. With the use of an intracavity type solid laser, green light having an optical output power of about 3 mW has been conventionally obtained by wavelength conversion with KTP (KTiOPO 4 ) when the semiconductor laser has a pumping intensity of 50 mW for Nd:YVO 4 . However, the half-width of the absorption spectrum of a laser material of Nd:YVO 4 is a few nm (nanometers). Therefore, presence of a mode hopping and multiplication of the longitudinal mode may cause output noises. This makes it mandatory to stabilize the oscillation wavelength of the semiconductor laser. In the field of light generating apparatuses incorporating semiconductor lasers capable of oscillation in a single longitudinal mode as pump light sources, there has been proposed a short wavelength light source apparatus in which a grating is provided by the end of a semiconductor laser through which laser light goes out. The general configuration of this conventional short wavelength light source apparatus will be described with reference to Figure 25 . In Figure 25 , reference numeral Q01 denotes a 60 mW-class AlGaAs semiconductor laser for the 809 nm band; Q02 denotes a collimating lens; Q03 denotes a focusing lens (f:14.5 mm); Q04 denotes a grating disposed at an angle of  with the optical axis of the semiconductor laser Q01 . The grating Q04 has a linear shape. A diffraction efficiency of about 10% was obtained under the following conditions: the incident angle was 30°; the depth was 0.29 µm; and the pitch was 0.83 µm. The grating Q04 has a wavelength dispersing effect, and therefore is capable of locking the oscillation wavelength of the semiconductor laser Q01 by feeding back light of a certain wavelength to the semiconductor laser Q01 as a first-order diffracted light. Laser light (zero-order diffracted light) reflected from the grating Q04 is lead through the focusing lens Q03 so as to be focused on an end face Q08 of a laser medium of Nd:YVO 4 Q07 . The fundamental wave, resonated between an output mirror Q09 and the end face Q08 of the Nd:YVO 4 laser medium Q07 is subjected to wavelength conversion by a non-linear device optical crystal of KTP(KTiOPO 4 ) Q10 so as to be output through the output mirror Q09 . As is seen from Figures 13 and 25 , when attempting to stabilize the longitudinal mode oscillation of a semiconductor laser with the use of a grating, the stability may be drastically lowered due to temporal change of the grating and/or the posture in which it is maintained and change in the surrounding temperature. The grating angle dependence of the wavelength stability is 28 nm/deg. Moreover, the whole module (apparatus) cannot be configurated in a linear shape because the optical axis of the semiconductor laser is bent with respect to the optical axis of the emitted light. Therefore, miniaturization of such a short wavelength light source apparatus becomes difficult. Furthermore, use of a grating for adjustments against changes in the refractive index due to temperature changes of the optical wavelength converting element and/or optical damage, that is, changes in the phase matching wavelength, requires a complicated and high-precision adjustment mechanism. US-A-5185752 defines a short wavelength light source apparatus according to the preamble of claims 1, 9, 27 and 45. SUMMARY OF THE INVENTION A short wavelength light source apparatus according to the present invention is defined by the features of independent claims 1, 9, 27 and 45. Preferred embodiments are defined by the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a view showing a short wavelength light source apparatus according to the present invention incorporating a semiconductor laser as a pump light source, a polarization inversion type waveguide element, and a thin film optical element (reflective type filter) stabilizing the oscillation wavelength of the semiconductor laser. Figure 2 is a graph showing a reflection spectrum of a thin film optical element (reflective type filter) according to the present invention. Figure 3 is a view showing a short wavelength light source apparatus according to the present invention incorporating a semiconductor laser as a pump light source, a polarization inversion type waveguide element, and a thin film optical element (transmission type filter) stabilizing the oscillation wavelength of the semiconductor laser. Figure 4 is a graph showing a reflection spectrum of a thin film optical element (transmission type filter) according to the present invention. Figure 5 is a view showing a short wavelength light source apparatus according to the present invention incorporating a semiconductor laser as a pump light source, a polarization inversion type waveguide element, and a thin film optical element (transmission type filter) stabilizing the oscillation wavelength of the semiconductor laser. Figure 6 is a view showing a short wavelength light source apparatus according to the present invention incorporating a semiconductor laser as a pump light source, a polarization inversion type waveguide element, and a thin film optical element (transmission type filter) stabilizing the oscillation wavelength of the semiconductor laser. Figure 7 is a view showing a short wavelength light source apparatus according to the present invention incorporating a semiconductor laser as a pump light source, a polarization inversion type bulk element, and a thin film optical element (transmission type filter) stabilizing the oscillation wavelength of the semiconductor laser. Figure 8 is a view showing a short wavelength light source apparatus according to the present invention incorporating a semiconductor laser as a pump light source, a polarization inversion type bulk element, and a thin film optical element (transmission type filter) stabilizing the oscillation wavelength of the semiconductor laser. Figure 9A is a view showing a short wavelength light source apparatus according to the present invention in which a dielectric multilayer film is provided on an outgoing end face of a semiconductor laser. Figure 9B is a view showing a short wavelength light source apparatus according to the present invention in which a dielectric multilayer film is provided on an end face of a polarization inversion type waveguide element. Figure 10 is a view showing a short wavelength light source apparatus according to the present invention in which a dielectric multilayer film is provided on an end face of a polarization inversion type waveguide element. Figure 11A is a schematic view showing a short wavelength light source apparatus according to the present invention including polarization-inverted layers having a constant polarization inversion period. Figure 11B is a schematic view showing a short wavelength light source apparatus according to the present invention including polarization-inverted layers having a polarization inversion period of a split structure. Figure 11C is a schematic view showing a short wavelength light source apparatus according to the present invention including polarization-inverted layers having a polarization inversion period of a chirp structure. Figure 12 is a view showing a short wavelength light source apparatus according to the present invention incorporating a semiconductor laser as a pump light source for exciting a polarization inversion type waveguide element having a polarization inversion period of a split structure, and a thin film optical element stabilizing the oscillation wavelength of the semiconductor laser. Figure 13 is a view showing a conventional short wavelength light source apparatus incorporating a semiconductor laser and a polarization inversion type waveguide element, in which grating feedback technique is used. Figure 14 is a graph showing the respective output power stabilities of a short wavelength light source apparatus according to the present invention incorporating a transmission type filter, and a conventional short wavelength light source apparatus in which grating feedback technique is used. Figure 15 is a view showing a conventional short wavelength light source apparatus incorporating a semiconductor laser and an intracavity type solid laser, in which grating feedback technique is used. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described by way of examples, with reference to the accompanying drawings. Example 1 Figure 1 schematically shows a configuration for a short wavelength light source apparatus according to a first example of the present invention. The short wavelength light source apparatus includes a semiconductor laser 101 including an active layer (not shown) for emitting laser light, an optical wavelength converting element 106 for receiving at least a portion of the generated laser light so as to generate short wavelength light (harmonics) having a shorter wavelength than that of the laser light. The semiconductor laser 101 is a 150 mW-class single longitudinal mode laser with an oscillation wavelength of the 870 nm band. The optical wavelength converting element 106 is a polarization inversion type waveguide element. This short wavelength light source apparatus includes a thin film optical element 103 in addition to an ordinary optical system including lenses, etc. The thin film optical element 103 selectively reflects a portion of laser light generated by the semiconductor laser 101 that belongs to a predetermined wavelength band so as to feed back the reflected light to the active layer of the semiconductor laser 101 . The thin film optical element 103 may be, for example, a band-stop type (reflective type) filter formed of a dielectric multilayer film, and can be obtained by forming a layer of dielectric material such as SiO 2 and TiO 2 on a glass substrate having a thickness of 0.5 mm. Figure 2 shows reflection characteristics of the thin film optical element 103 . As is seen from Figure 2 , the reflectance of the thin film optical element 103 has a peak of about 10%. Light in the vicinity of the 870 nm band is optically fed back to the active layer of the semiconductor laser 101 by the thin film optical element 103 . The thin film optical element 103 feeds back a relatively wide spectrum width of about 5 nm of reflected light to the active layer of the semiconductor laser 101 . According to the short wavelength light source apparatus of the present example, as is shown in Figure 1 , light generated by the semiconductor laser 101 is collimated by a lens 102 (N.A.: 0.55), and then is incident to the thin film optical element 103 . The thin film optical element 103 is disposed perpendicularly to the optical axis of the semiconductor laser 101 . A portion (about 10% or less) of light incident to the thin film optical element 103 that is in the vicinity of the 870 nm band is optically fed back to the active layer of the semiconductor laser 101 . The rest of the laser light, i.e. a portion of the laser light which is not reflected by the thin film optical element 103 , is transmitted through the thin film optical element 103 . As is shown in Figure 2 , the thin film optical element 103 feeds back a relatively wide spectrum width of about 5 nm of reflected light to the active layer of the semiconductor laser 101 . However, since the gain of a semiconductor laser depends largely on the wavelength, the semiconductor laser 101 of the present example achieved a stable oscillation in a single longitudinal mode by feeding back the reflected light having the above-mentioned spectrum, and the oscillation wavelength was stabilized at 870 nm. In the present example, the polarization direction of the laser light transmitted through the thin film optical element 103 is rotated by 90° around the optical axis by means of a λ/2 plate 104 . The laser light, after being transmitted through the λ/2 plate 104 , is focused on an incident end face of the polarization inversion type waveguide element (optical wavelength converting element) 106 by a focusing lens 105 . Thereafter, the laser light having a wavelength of 870 nm is converted into blue light having a wavelength of 435 nm while being propagated through a waveguide 108 . As a result, blue light is stably emitted from an end face of the polarization inversion type waveguide element 106 (hereinafter, this end face of a given waveguide will be referred to as an 'outgoing end face'). Hereinafter, the polarization inversion type waveguide element 106 will be described. The polarization inversion type waveguide element 106 of the present example includes a LiTaO 3 substrate 109 , an optical waveguide 108 (width: 4 µm) formed on the LiTaO 3 substrate 109 , and polarization-inverted layers 107 (period: 4 µm). The polarization inversion type waveguide element 106 is formed in the following manner. First, a Ta film is deposited on the LiTaO 3 , and thereafter is patterned into a periodic pattern by using a usual photoprocess and a dry etching method, whereby a plurality of slits are formed in the Ta film. Next, the LiTaO 3 substrate is subjected to a proton exchanging process for 30 minutes at 260°C, so as to form a proton exchanged layer (thickness: 0.8 µm) under the slits in the Ta layer. Thereafter, the LiTaO 3 substrate 109 is subjected to a heat treatment for 10 minutes at 590°C. The temperature during the heat treatment is increased at a rate (elevation rate) of 10°C/min., and is decreased at a rate (cooling rate) of 50°C/min. The Li concentration in regions directly below the proton exchanged layer of the LiTaO 3 109 decreases, so that the Curie temperature in such regions decreases. As a result, the regions under the proton exchanged layer have the capability of locally conducting polarization inversion. Thus, the polarization-inverted layers 107 are formed in the LiTaO 3 substrate 109 . Next, the LiTaO 3 substrate is subjected to an etching process for 2 minutes with the use of a solution of HF and HNF 3 mixed at a ratio of 1:1 so as to remove the Ta film. Then, the optical waveguide 108 is formed in the polarization-inverted layers 107 by a proton exchange process. Ta is patterned to a stripe shape so as to form a Ta mask for forming the optical waveguide 108 . The Ta mask thus obtained has slits each having a width of 4 µm and a length of 12 mm. The LiTaO 3 substrate 109 , which is covered with the above-mentioned Ta mask, is subjected to a proton exchange process for 16 minutes at 260°C so as to form high refractive index layer having a thickness of 0.5 µm, and is subjected to a heat treatment for 10 minutes at 380 °C after removing the Ta mask. The regions directly below the slits of the Ta mask, where proton exchange has been conducted, form the optical waveguide layer 108 of which refractive index has been increased by about 0.03. Since the refractive index of the LiTaO 3 substrate 109 is about 2.2, Fresnel reflection of about 14% is inevitably generated unless a coating is provided on end faces thereof. If Fresnel reflection is present, there emerges light returning to the semiconductor laser 101 , thereby hindering stabilization of the oscillation wavelength by the thin film optical element 103 . Therefore, it is preferable to form an AR coating which does not reflect light having a wavelength of 870 nm on the incident end face and the outgoing end face of the polarization inversion type waveguide element 106 . The λ/2 plate 104 inserted in the optical system composed essentially of the collimating lens 102 and the focusing lens 105 may alternatively be formed on the thin film optical element 103 . In that case, the number of the component elements of the short wavelength light source apparatus would be reduced, thereby facilitating further miniaturization of the short wavelength light source apparatus. Example 2 Although the polarization inversion type waveguide element 106 including a substrate composed of LiTaO 3 (LT) is used as an optical wavelength converting element in the short wavelength light source apparatus shown in Figure 1 , similarly stable short wavelength light can be obtained by using: a polarization inversion type waveguide element including a substrate composed of LiNbO 3 (LN) or KTiOPO 4 (KTP); a non-linear optical crystal having a large non-linear optical constant, e.g. KNbO 3 (KN); or a polarization inversion type bulk element including a substrate composed of LT, LN, or KTP. An AR coating which does not reflect light having a wavelength of 870 nm is provided on an incident end face and an outgoing end face of the optical wavelength converting element according to the present example. A method for fabricating a polarization inversion type bulk element having deep polarization-inverted layers will be described. Deep polarization-inverted layers can be fabricated by a drawing method using an electron beam (EB), a focusing ion beam (FIB) method, etc., and by a proton exchange method. A polarization inversion type bulk element can be fabricated by any of the following four methods. (1) A method using EB irradiation i) A substrate composed of a dielectric material of LiNbO 3 or LiTaO 3 is irradiated with charged particles (electrons) accelerated at an acceleration voltage in the range of 10 to 100 keV in such a manner that the current density at a surface of the substrate becomes 1 to 1000 µA/mm 2 while applying an electric field of 10 V/mm to 100kV/mm. Thus, deep polarization-inverted layers can be obtained. (2) A method using EB irradiation ii) A metal film is vapor deposited on a +C plane of a substrate composed of a dielectric material of LiNbO 3 or LiTaO 3 (C plates), and is grounded. An electron beam accelerated at an acceleration voltage of 25 keV is focused so as to be radiated on each substrate from a -C plane thereof so that the polarization at the irradiated portion is inverted. The polarization-inverted layers thus obtained reach the bottom face of the substrate (thickness: 0.5 mm). Thus, deep polarization-inverted layers can be obtained. (3) A method using FIB irradiation By using a liquid metal source of AuSi as an ion source, Si 2+ ions are focused and radiated on a substrate, the Si 2+ ions having been separated and selected by means of an electric field filter. The focused area extends about 1 µm ø. The substrate is a C plate composed of LiTaO 3 , which is grounded to a sample holder with metal paste. The Si 2+ ions are focused on the +C plane of the substrate. A periodic scanning is conducted with the focused ions while utilizing computer control. The above-mentioned irradiation is conducted under the conditions that: the acceleration energy is 200 keV; the amount of current is 120 pA; and the scanning velocity is 84 µm/sec. The polarization-inverted layers thus obtained reach the bottom face of the substrate, and has a width of 1.8 µm. The width of each polarization-inverted layer is uniform along the depth direction of the polarization-inverted layer. On forming periodic polarization-inverted layers (period:4 µm) under the same conditions as above, a uniform periodic structure having a period of 4 µm was formed, with the width and the depth of each polarization-inverted layer being 1.8 µm and 0.5 mm, respectively. (4) A method using proton exchange technique Ta is vapor deposited on a -C plane of a LiTaO 3 substrate and is subjected to a photolithography and a dry etching so as to form stripes having a period of 4 µm. Several drops of pyrophoric acid are placed on the stripes, and a heat treatment is conducted for 30 minutes on a hot plate heated at 230°C. Thus, deep polarization-inverted layers can be obtained. By any of the above mentioned methods (1) to (4), deep polarization-inverted layers are formed on an LiTaO 3 substrate. An incident end face and an outgoing end face are optically polished, and are each coated with an AR coating which does not reflect light having a wavelength of 870 nm, whereby a polarization inversion type bulk element according to a second example of the present invention is formed. Thus, a short wavelength light source apparatus is realized with the polarization inversion type bulk element being incorporated instead of the polarization inversion type waveguide element 106 shown in Figure 1 . Example 3 Hereinafter, a short wavelength light source apparatus according to a third example of the present invention will be described with reference to Figure 3 . The short wavelength light source apparatus shown in Figure 3 includes a semiconductor laser 301 including an active layer (not shown) for emitting laser light, an optical wavelength converting element 306 for receiving at least a portion of the generated laser light so as to generate short wavelength light (harmonics) having a shorter wavelength than that of the laser light. The semiconductor laser 301 is a 150 mW-class single longitudinal mode laser with an oscillation wavelength of the 870 nm band. The optical wavelength converting element 306 is a polarization inversion type waveguide element. This short wavelength light source apparatus includes a thin film optical element 303 in addition to an ordinary optical system including lenses, etc. The thin film optical element 303 selectively transmits a portion of laser light generated by the semiconductor laser 301 that belongs to a predetermined wavelength band. A portion of the transmitted light is fed back to the active layer of the semiconductor laser 301 , as is described later in the present specification. The thin film optical element 303 may be, for example, a band-pass type (transmission type) filter formed of a dielectric multilayer film, and can be obtained by forming a few dozens of layers of dielectric material such as TiO 2 on a glass substrate having a thickness of 0.5 mm. Figure 4 shows transmission characteristics of the thin film optical element 303 . As is seen from Figure 4 , the reflectance of the thin film element 303 has a maximum transmittance of about 80% at a wavelength of 870 nm. The half-width of the transmission spectrum of the thin film element 303 is 1 nm. The transmittance of the thin film optical element 303 has angle dependence. The transmission spectrum shown in Figure 4 represents a spectrum in the case where the principal plane of the thin film optical element 303 is disposed at an angle (i.e. incident angle) of 20° with the optical axis. When the incident angle is 20°, the shift amount of the transmission center wavelength of the thin film optical element 303 has an angle dependence of 1.5 nm/deg; when the incident angle is in the vicinity of 10°, the shift amount of the transmission center wavelength has an angle dependence of 0.9 nm/deg; and when the incident angle is 0°, the shift amount of the transmission center wavelength has substantially no angle dependence. According to the short wavelength light source apparatus of the present example, as is shown in Figure 3 , light generated by the semiconductor laser 301 is collimated by a lens 302 , and then is incident to the thin film optical element 303 . The thin film optical element 303 is disposed obliquely with respect to the optical axis of the semiconductor laser 301 . According to the present example, only light having a wavelength in the vicinity of 870 nm is selectively transmitted through the thin film optical element 303 when the thin film optical element 303 is disposed at an angle of 20° with the optical axis of the semiconductor laser 301 . The rest of the laser light, i.e. a portion of the laser light which is not transmitted through the thin film optical element 303 , is reflected by the thin film optical element 303 . The reflected light is not fed back to the active layer of the semiconductor laser 301 , since the thin film optical element 303 is disposed obliquely with respect to the optical axis of the semiconductor laser 301 . In the present example, the polarization direction of the laser light transmitted through the thin film optical element 303 is rotated by 90° around the optical axis by means of a λ/2 plate 304 . The laser light, after being transmitted through the λ/2 plate 304 , is focused on an incident end face 307 of the polarization inversion type waveguide element (optical wavelength converting element) 306 by a focusing lens 305 . An AR coating is provided on the incident end face 307 of the polarization inversion type waveguide element 306 ; otherwise, Fresnel reflection of about 14% would inevitably be generated. Since light generated by the semiconductor laser 301 is focused on the incident end face 307 of the polarization inversion type waveguide element 306 , an outgoing end face of the semiconductor laser 301 and the incident end face 307 of the polarization inversion type waveguide element 306 are confocally disposed. As a result, a portion of light generated by the semiconductor laser 301 is reflected by the incident end face 307 of the polarization inversion type waveguide element 306 so as to be fed back to the active layer of the semiconductor laser 301 . As is described above, by disposing the thin film optical element 303 between the semiconductor laser 301 and the polarization inversion type waveguide element 306 at an angle of 20° with the optical axis, only light having a wavelength in the vicinity of 870 nm is incident to and reflected by the incident end face 307 , so as to be optically fed back to the semiconductor laser 301 . As a result, the semiconductor laser 301 achieves stable oscillation in a single longitudinal mode, with the oscillation wavelength being stabilized at 870 nm. The light (wavelength: 870 nm) emitted from the incident end face 307 of the polarization inversion type waveguide element 306 and coupled to a waveguide of the polarization inversion type waveguide element 306 is converted into blue light having a wavelength of 435 nm while being propagated through the waveguide. As a result, blue light is stably emitted from an outgoing end face 308 of the polarization inversion type waveguide element 306 . A portion of the light coupled to the above-mentioned waveguide is Fresnel reflected by the outgoing end face 308 . Since it is preferable to restrain light having a wavelength of 870 nm from being reflected by the outgoing end face 308 , an AR coating which does not reflect the fundamental wave (wavelength: 870 nm) is provided on the outgoing end face 308 . The polarization inversion type waveguide element 306 includes polarization-inverted layers having a period of 4 µm and an optical waveguide having a width of 4 µm and formed in an LiTaO 3 substrate, as in the case of the polarization inversion type waveguide element 106 in Example 1. By adjusting the temperature of the polarization inversion type waveguide element 306 so that the phase matching wavelength is tuned to be 870 nm, the fundamental wave (870 nm) is converted into light having a wavelength of 435 nm. As a result, blue light is obtained through the outgoing end face 308 of the polarization inversion type waveguide element 306 . The polarization inversion type waveguide element 306 has a length of 10 mm and a conversion efficiency of 200%/W. As a result, a harmonic wave of about 10 mW is obtained, the fundamental wave power of about 70 mW being coupled to the waveguide. It is preferable to provide an AR coating which does not reflect the 430 nm harmonic wave on the outgoing end face 308 so that the harmonic light can be obtained at even higher efficiency. Figure 14 shows temporal change in the output power of the short wavelength light source apparatus of the present example in comparison with a case where a grating is employed. It will be seen from Figure 14 that the polarization inversion type waveguide element 306 is stable as a function of time as well, since the wavelength shifts by a smaller amount depending on the angle when using a filter than when using a grating. By wavelength locking using such a thin film optical element 303 , the wavelength does not shift depending on the angle by such a large amount as 28 nm/deg as in the case where a grating is used. Therefore, the adjustment using the thin film optical element 303 is facilitated, and the short wavelength light source apparatus is made stable against temporal deterioration even if configurated as a module. Moreover, the short wavelength light source apparatus of the present example is stable against temperature changes as well (wavelength shift against temperature changes: 0.005 nm/°C). The short wavelength light source apparatus is also stable against changes in moisture. Usually, an AR coating which does not reflect laser light generated from a semiconductor laser is provided on an end face of a polarization inversion type waveguide element. However, it is difficult to maintain the reflectance of the end face below about 0.1%. Even if the reflectance of the end face is reduced to about 0.1%, the longitudinal mode of the semiconductor laser may become unstable due to a small amount of light reflected by the end face, which amount of light is fed back to the semiconductor laser. The present example effectively utilizes the returned light due to the reflection so as to stabilize the oscillation of the semiconductor laser. Therefore, the present example has considerable practical significance. Although Fresnel reflection at the incident end face of the polarization inversion type waveguide element is utilized in the present example, it is also applicable to optimize the reflection amount by providing an appropriate reflection coating on the incident end face so as to adjust the reflection amount at the incident end face. The higher the reflectance of such a reflection coating is, the more stable the oscillation wavelength of the semiconductor laser becomes. However, the higher the reflectance of the reflection coating is, the smaller amount of light is coupled to the waveguide, resulting in a lower output power of the harmonic light. If a reflection coating having a reflectance of about 5%, on the other hand, the output power of the harmonic light increases due to an increase in the amount of light coupled to the waveguide. The λ/2 plate 304 disposed in the optical system (including the collimating lens 302 and the focusing lens 305 ) may alternatively be formed on the thin film optical element 303 . In that case, the number of the component elements of the short wavelength light source apparatus would be reduced, thereby facilitating further miniaturization of the short wavelength light source apparatus. Moreover, the peak wavelength of the transmission spectrum of the thin film optical element 303 has angle dependence. By mounting a rotation mechanism on the thin film optical element 303 and adjusting the angle of the thin film optical element 303 with respect to the laser light by means of the rotation mechanism, it becomes possible to control the peak wavelength of the transmission spectrum at a high accuracy. By using such a rotation mechanism, it becomes possible to monitor the output power of harmonic wave obtained from the outgoing end face 308 of the polarization inversion type waveguide element 306 . Specifically, the angle which the thin film optical element 303 constitutes with the laser light is adjusted by monitoring the output power of harmonic wave obtained from the outgoing end face 308 of the polarization inversion type waveguide element 306 so that the monitored output power is maximized. By making such an adjustment, the oscillation wavelength of the semiconductor laser 301 can be made equal to the phase matching wavelength of the polarization inversion type waveguide element 306 . By further providing a feedback circuit for controlling the rotation mechanism in addition to a mechanism for monitoring the output power of the harmonic wave, stable harmonic wave output can be obtained. By providing such a rotation mechanism for the thin film optical element 303 , the oscillation wavelength of the semiconductor laser 301 can be stabilized against changes in the refractive index of the substrate of the polarization inversion type waveguide element 306 due to changes in the temperature of the thin film optical element 303 and/or optical damage (photorefractive effect), so that stable output power of the harmonic wave can be obtained. As the rotation mechanism, a rotation monitor and/or an actuator may be used. It is also applicable to use a piezo element on a fulcrum so that the piezo element has a rotatory action. Moreover, only one thin film optical element 303 is disposed between the semiconductor laser 301 and the polarization inversion type waveguide element 306 in the present example, a plurality of thin film optical elements may alternatively be inserted therein, so that the half width of the transmission spectrum shown in Figure 4 becomes substantially narrow, thereby improving the wavelength selectivity. Example 4 Although the polarization inversion type waveguide element 306 including a substrate composed of LiTaO 3 (LT) is used as an optical wavelength converting element in the short wavelength light source apparatus shown in Figure 3 , similarly stable short wavelength light can be obtained by using: a polarization inversion type waveguide element including a substrate composed of LiNbO 3 (LN) or KTiOPO 4 (KTP); an organic or inorganic non-linear optical crystal (a ring resonator structure, phase matching type waveguide device, etc.) having a large non-linear optical constant, e.g. KNbO 3 (KN); or a polarization inversion type bulk element including a substrate composed of LT, LN, or KTP described in Example 2. In this case, too, it is preferable to provide an AR coating which reflects neither light having a wavelength of 870 nm nor light having a wavelength of 435 nm on an incident end face and an outgoing end face of the optical wavelength converting element. Example 5 Hereinafter, a short wavelength light source apparatus according to a fifth example of the present invention will be described with reference to Figure 5 . The short wavelength light source apparatus shown in Figure 5 includes a semiconductor laser 501 including an active layer (not shown) for emitting laser light, an optical wavelength converting element 506 for receiving at least a portion of the generated laser light so as to generate short wavelength light (harmonics) having a shorter wavelength than that of the laser light. The semiconductor laser 501 is a 150 mW-class single longitudinal mode laser with an oscillation wavelength of the 870 nm band. The optical wavelength converting element 506 is a polarization inversion type waveguide element. This short wavelength light source apparatus includes a thin film optical element 503 having the same structure as that of the thin film optical element 303 in Example 3. Accordingly, the thin film optical element 503 has the transmission characteristics shown in Figure 4. The present example is characterized in that Fresnel reflection occurring at an outgoing end face 508 of the polarization inversion type waveguide element 506 is utilized as returned light to the semiconductor laser 501 . An incident end face of the polarization inversion type waveguide element 506 has a low reflectance. It is preferable to provide an AR coating on the incident end face 507 . Light emitted from the semiconductor laser 501 is focused on the incident end face of the polarization inversion type waveguide element 506 so as to be coupled to an optical waveguide of the polarization inversion type waveguide element 506 . An outgoing end face of the semiconductor laser 501 and the incident end face 507 of the polarization inversion type waveguide element 506 are confocally disposed. The outgoing end face of the semiconductor laser 501 also constitutes a confocal system with the outgoing end face 508 of the polarization inversion type waveguide element 506 . As a result, light reflected by the outgoing end face 508 is all optically fed back to the active layer of the semiconductor laser 501 . If the thin film optical element 503 is not provided, the above-mentioned reflected light (returned light) may cause the semiconductor laser 501 to have mode hopping or to have a multitude of modes. By disposing the thin film optical element 503 between the semiconductor laser 501 and the polarization inversion type waveguide element 506 at an angle of 20° with the optical axis, it is ensured that only light having a wavelength in the vicinity of 870 nm is incident to the incident end face 507 and coupled to the optical waveguide, so that only light having a wavelength in the vicinity of 870 nm which is reflected by the outgoing end face 508 of the polarization inversion type waveguide element 506 is optically fed back to the semiconductor laser 501 . As a result, the longitudinal mode of the semiconductor laser 501 is stabilized as a single mode in the vicinity of 870 nm. By providing a high reflectance (HR) coating for reflecting light having a wavelength of 870 nm (preferably at a 100% reflectance) on the outgoing end face 508 , the amount of light returning to the semiconductor laser 501 can be increased, so as to further stabilize the operation of the short wavelength light source apparatus. As is seen from Figure 5 , the thin film optical element 503 is disposed at an angle with the optical axis of the semiconductor laser 501 . The thin film optical element 503 reflects light having wavelengths other than those in the vicinity of 870 nm. In other words, light having wavelengths other than those in the vicinity of 870 nm is not optically fed back to the active layer of the semiconductor laser 501 . By so adjusting the temperature of the polarization inversion type waveguide element 506 that the phase matching wavelength is tuned to be 870 nm, the fundamental wave (870 nm) coupled to the polarization inversion type waveguide element 506 is converted into light having a wavelength of 435 nm. As a result, blue light is obtained through the outgoing end face 508 of the polarization inversion type waveguide element 506 . The temperature-based tuning may be realized by, although not shown in Figure 5, fixing the polarization inversion type waveguide element 506 on a Peltier element, or providing a heater on the optical waveguide. The polarization inversion type waveguide element 506 has a length of 10 mm and a conversion efficiency of 200%/W. As a result, a harmonic wave of about 10 mW is obtained, the fundamental wave power of about 70 mW being coupled to the waveguide. It is preferable to provide an AR coating which does not reflect the 435 nm harmonic wave on the outgoing end face 508 of the polarization inversion type waveguide element 506 so that the harmonic light can be obtained at even higher efficiency. By providing an HR coating for reflecting harmonic waves on the incident end face 507 of the polarization inversion type waveguide element 506 , the harmonic wave obtained by converting the light fed back from the outgoing end face 508 to the semiconductor laser 501 is also reflected by the incident end face 507 so as to be obtained through the outgoing end face 508 , thereby achieving an even higher efficiency. By further providing a coating having a reflectance of 97% and a coating having a reflectance of 99.9% on the incident end face 507 and the outgoing end face 508 , respectively, a waveguide-type external resonator can be constructed, which would increase the conversion efficiency to be twenty times as large. Usually, an AR coating which does not reflect laser light generated from a semiconductor laser is provided on an end face of a polarization inversion type waveguide element. However, it is difficult to maintain the reflectance of the end face below about 0.1%. Even if the reflectance of the end face is reduced to about 0.1%, the longitudinal mode of the semiconductor laser may become unstable due to a small amount of light reflected by the end face, which amount of light is fed back to the semiconductor laser. The present example effectively utilizes the returned light due to the reflection so as to stabilize the longitudinal mode of the semiconductor laser. Therefore, the present example has considerable practical significance. The λ/2 plate 504 disposed in the optical system (including a collimating lens 502 and a focusing lens 505 ) may alternatively be formed on the thin film optical element 503 . In that case, the number of the component elements of the short wavelength light source apparatus would be reduced, thereby facilitating further miniaturization of the short wavelength light source apparatus. Moreover, by providing a rotation mechanism and a feedback circuit for the thin film optical element 503 , the harmonic output power can be stabilized. Example 6 Although the polarization inversion type waveguide element 506 including a substrate composed of LiTaO 3 (LT) is used as an optical wavelength converting element in the short wavelength light source apparatus shown in Figure 5, similarly stable short wavelength light can be obtained by using a polarization inversion type waveguide element including a substrate composed of LiNbO 3 (LN) or KTiOPO 4 (KTP), a phase matching type waveguide device composed of KN, etc., or the like. In this case, too, it is preferable to provide an AR coating which does not reflect light having a wavelength of 870 nm and an HR coating for reflecting light having a wavelength of 870 nm on, respectively, an incident end face and an outgoing face of the optical wavelength converting element. Example 7 Hereinafter, a short wavelength light source apparatus according to a seventh example of the present invention will be described with reference to Figure 6 . The short wavelength light source apparatus of the present example incorporates a thin film optical element 603 (band-pass type filter) having a transmission spectrum shown in Figure 4 . A polarization inversion type waveguide element 606 is used as an optical wavelength converting element. The semiconductor laser 601 is a 150 mW-class single longitudinal mode laser with an oscillation wavelength of the 870 nm band. Reference numeral 602 denotes a collimating lens (N.A.: 0.55); 604 denotes a λ/2 plate; and 605 denotes a focusing lens for coupling light to the waveguide. A wavelength selection mirror 607 is an optical element for separating a harmonic wave obtained by wavelength conversion from the fundamental light. The polarization inversion type waveguide element 606 includes polarization-inverted layers (period: 4 µm) formed in a LiTaO 3 substrate, and an optical waveguide (width: 4 µm) as in Example 1. In the present example, an HR coating for reflecting the fundamental wave (wavelength: 870 nm) and a harmonic wave (wavelength: 435 nm) is provided on an end face 609 of the polarization inversion type waveguide element 606 . An AR coating which reflects neither light having a wavelength of 870 nm nor light having a wavelength of 435 nm is provided on an incident end face 608 of the polarization inversion type waveguide element 606 so as to reduce the reflectance of the incident end face 608 . Laser light emitted from the semiconductor laser 601 is transmitted through the thin film optical element 603 , and then focused on the incident end face 608 of the polarization inversion type waveguide element 606 by the focusing lens 605 so as to be coupled to the waveguide. An outgoing end face of the semiconductor laser 601 and the incident end face 608 of the polarization inversion type waveguide element 606 are confocally disposed. The outgoing end face of the semiconductor laser 601 also constitutes a confocal system with the end face 609 of the polarization inversion type waveguide element 606 . As a result, light reflected by the end face 609 is all optically fed back to an active layer of the semiconductor laser 601 . If the thin film optical element 603 is not provided, the above-mentioned reflected light (returned light) may cause the semiconductor laser 601 to have mode hopping or to have a multitude of modes. By disposing the thin film optical element 603 between the semiconductor laser 601 and the polarization inversion type waveguide element 606 at an angle of 20° with the optical axis, it is ensured that only light having a wavelength in the vicinity of 870 nm is incident to the incident end face 608 and coupled to the optical waveguide, so that only light having a wavelength in the vicinity of 870 nm which is reflected by the end face 609 is transmitted back through the thin film element 603 so as to be optically fed back to the semiconductor laser 601 . As a result, the longitudinal mode of the semiconductor laser 601 is stabilized as a single mode in the vicinity of 870 nm. Moreover, according to the present example, an HR coating for reflecting light having a wavelength of 870 nm and light having a wavelength of 435 nm (preferably at a 100% reflectance) is provided on the end face 609 . As a result, the fundamental wave and the harmonic wave interact with each other also when being propagated backwards, i.e. from the end face 609 to the semiconductor laser 601 . Therefore, the inter-action length obtained by the polarization inversion type waveguide element 606 becomes twice as large, so that the conversion efficiency into the harmonic wave becomes four times as large. By so adjusting the temperature of the polarization inversion type waveguide element 606 that the phase matching wavelength is tuned to be 870 nm, as in Example 5, the fundamental wave (870 nm) coupled to the polarization inversion type waveguide element 606 is converted into a harmonic wave having a wavelength of 435 nm. As a result, blue light goes out through the incident end face 608 of the polarization inversion type waveguide element 606 . The blue light is separated from the fundamental wave by the wavelength selection mirror 607 . The polarization inversion type waveguide element 606 has a length of 10 mm and a conversion efficiency of 800%/W. As a result, a harmonic wave of about 30 mW is obtained, the fundamental wave power of about 70 mW being coupled to the waveguide. According to the short wavelength light source apparatus of the present example, the reflectance of the end face 609 is made large. As a result, not only that the harmonic light is obtained with higher stability but also the interaction length of the optical wavelength converting element (polarization inversion type waveguide element) 606 is increased. Therefore, the present example has considerable practical significance. The λ/2 plate 604 disposed in the optical system (including the collimating lens 602 and the focusing lens 605 ) may alternatively be formed on the thin film optical element 603 . In that case, the number of the component elements of the short wavelength light source apparatus may be reduced, thereby facilitating further miniaturization of the short wavelength light source apparatus. The number of the component elements of the short wavelength light source apparatus may be further reduced and further miniaturization of the short wavelength light source apparatus would be facilitated by forming the wavelength selection mirror 607 disposed in the optical system on the thin film optical element 603 . Moreover, the harmonic output power can be stabilized by providing a rotation mechanism and a feedback circuit for the thin film optical element 603 . Example 8 Although the polarization inversion type waveguide element 606 including a substrate composed of LiTaO 3 (LT) is used as an optical wavelength converting element in the short wavelength light source apparatus shown in Figure 6 , similarly stable short wavelength light can be obtained by using a polarization inversion type waveguide element including a substrate composed of LiNbO 3 (LN) or KTiOPO 4 (KTP), a phase matching type waveguide device composed of KN, etc., or the like. In this case, too, it is preferable to provide an AR coating which reflects neither light having a wavelength of 870 nm nor light having a wavelength of 435 nm on an incident face of the optical wavelength converting element and to provide an HR coating for reflecting light having a wavelength of 870 nm and light having a wavelength of 435 nm on an outgoing face of the optical wavelength converting element. Example 9 Hereinafter, a short wavelength light source apparatus according to a ninth example of the present invention will be described with reference to Figure 7 . The short wavelength light source apparatus of the present example incorporates a thin film optical element 703 (band-pass type filter) having a transmission spectrum shown in Figure 4 . A polarization inversion type bulk element 705 is used as an optical wavelength converting element. The semiconductor laser 701 is a 150 mW-class single longitudinal mode laser with an oscillation wavelength of the 870 nm band. Reference numeral 702 denotes a collimating lens (N.A.: 0.55); and 704 denotes a long-focus focusing lens (N.A.: 0.2) for focusing light on the polarization inversion type bulk element 705 . The polarization inversion type waveguide element 705 includes deep polarization-inverted layers (period: 4 µm) formed in a LiTaO 3 substrate by a similar manner to that used in Example 2. In the present example, the reflectance of an incident end face 706 of the polarization inversion type bulk element 705 is reduced, preferably by providing an AR coating on the incident end face 706 , so as to utilize light reflected by an outgoing end face 707 as light returning to the semiconductor laser 701 . Light emitted from the semiconductor laser 701 is focused on the outgoing end face 707 of the polarization inversion type bulk element 705 . An outgoing end face of the semiconductor laser 701 and the incident end face 706 of the polarization inversion type bulk element 705 are confocally disposed. As a result, light reflected by the outgoing end face 707 is all optically fed back to an active layer of the semiconductor laser 701 . If the thin film optical element 703 is not provided, the above-mentioned reflected light (returned light) may cause the semiconductor laser 701 to have mode hopping or to have a multitude of modes. By disposing the thin film optical element 703 between the semiconductor laser 701 and the polarization inversion type bulk element 705 at an angle of 20° with the optical axis, it is ensured that only light having a wavelength in the vicinity of 870 nm is incident to the polarization inversion type bulk element 705 , so that only light having a wavelength in the vicinity of 870 nm which is reflected by the outgoing end face 707 is transmitted back through the thin film element 703 so as to be optically fed back to the semiconductor laser 701 . As a result, the longitudinal mode of the semiconductor laser 701 is stabilized as a single mode in the vicinity of 870 nm. By providing an HR coating for reflecting light having a wavelength of 870 nm (preferably at a 100% reflectance) on the outgoing end face 707 , the amount of light returning to the semiconductor laser 701 can be increased, so as to further stabilize the operation of the short wavelength light source apparatus. The thin film optical element 703 is disposed obliquely with respect to the optical axis of the semiconductor laser 701 . The thin film optical element 703 reflects light having wavelengths other than those in the vicinity of 870 nm. In other words, light having wavelengths other than those in the vicinity of 870 nm is not optically fed back to the active layer of the semiconductor laser 701 . By adjusting the temperature of the polarization inversion type bulk element 705 so that the phase matching wavelength is tuned to be 870 nm, as in Example 5, the fundamental wave (870 nm) coupled to the polarization inversion type bulk element 705 is converted into a harmonic wave having a wavelength of 435 nm. As a result, blue light goes out through the outgoing end face 707 of the polarization inversion type bulk element 705 . The polarization inversion type bulk element 705 has a length of 2 mm and a conversion efficiency of 8%/W. As a result, a harmonic wave of about 0.4 mW is obtained, the fundamental wave power of about 70 mW being coupled to the waveguide. It is preferable to provide an AR coating which does not reflect the 475 nm harmonic wave on the outgoing end face 707 of the polarization inversion type bulk element 705 , so that the harmonic wave is obtained with a further increased efficiency. Usually, an AR coating which does not reflect laser light generated from a semiconductor laser is provided on an end face of a polarization inversion type bulk element. However, it is difficult to maintain the reflectance of the end face below about 0.1%. Even if the reflectance of the end face is reduced to about 0.1%, the longitudinal mode of the semiconductor laser may become unstable due to a small amount of light reflected by the end face, which amount of light is fed back to the semiconductor laser. The present example effectively utilizes the returned light due to the reflection so as to stabilize the longitudinal mode of the semiconductor laser. Therefore, the present example has considerable practical significance. An even higher harmonic output power can be obtained by using a 1 W-class single longitudinal mode laser including a light amplifying portion as the pump laser source. Moreover, the harmonic output power can be stabilized by providing a rotation mechanism and a feedback circuit for the thin film optical element 703 , as in Example 3. Example 10 Although the polarization inversion type waveguide element 705 including a substrate composed of LiTaO 3 (LT) is used as an optical wavelength converting element in the short wavelength light source apparatus shown in Figure 7 , similarly stable short wavelength light can be obtained by using: a polarization inversion type bulk element including a substrate composed of LiNbO 3 (LN) or KTiOPO 4 (KTP); or an organic or inorganic non-linear optical crystal (a ring resonator structure, phase matching type waveguide device, etc.) having a large non-linear optical constant, e.g. KNbO 3 (KN). In this case, too, it is preferable to provide an AR coating which does not reflect light having a wavelength of 870 nm on an incident end face of the optical wavelength converting element, and to provide an HR coating for reflecting light having a wavelength of 870 nm and an AR coating which does not reflect light having a wavelength of 435 nm on an outgoing end face of the optical wavelength converting element. Example 11 Hereinafter, a short wavelength light source apparatus according to an eleventh example of the present invention will be described with reference to Figure 8 . The short wavelength light source apparatus of the present example incorporates a thin film optical element 803 (band-pass type filter) having a transmission spectrum shown in Figure 4 . A polarization inversion type bulk element 805 is used as an optical wavelength converting element. The semiconductor laser 801 is a 150 mW-class single longitudinal mode laser with an oscillation wavelength of the 870 nm band. Reference numeral 802 denotes a collimating lens (N.A.: 0.55); and 804 denotes a long-focus focusing lens (N.A.: 0.2) for focusing light on the polarization inversion type bulk element 805 . A wavelength selection mirror 806 is an optical element for separating a harmonic wave obtained by wavelength conversion from the fundamental light. The polarization inversion type waveguide element 805 includes deep polarization-inverted layers (period: 4 µm) formed in a LiTaO 3 substrate by a similar manner to that used in Example 2. An HR coating for reflecting light having a wavelength of 870 nm and light having a wavelength of 435 nm is provided on an end face 808 of the polarization inversion type bulk element 805 . The reflectance of an incident end face 807 of the polarization inversion type bulk element 805 is reduced by providing an AR coating which reflects neither light having a wavelength of 870 nm nor light having a wavelength of 435 nm thereon. Light emitted from the semiconductor laser 801 is led through the thin film optical element 803 so as to be focused by the focusing lens 804 on the end face 808 of the polarization inversion type bulk element 805 . An outgoing end face of the semiconductor laser 801 and the end face 808 of the polarization inversion type bulk element 805 are confocally disposed. As a result, light reflected by the end face 808 is optically fed back to an active layer of the semiconductor laser 801 . If the thin film optical element 803 is not provided, the above-mentioned reflected light (returned light) may cause the semiconductor laser 801 to have mode hopping or to have a multitude of modes. By disposing the thin film optical element 803 between the semiconductor laser 801 and the polarization inversion type bulk element 805 at an angle of 20° with the optical axis, it is ensured that only light having a wavelength in the vicinity of 870 nm is incident to the polarization inversion type bulk element 805 , so that only light having a wavelength in the vicinity of 870 nm which is reflected by the end face 808 is transmitted back through the thin film element 803 so as to be optically fed back to the semiconductor laser 801 . As a result, the longitudinal mode of the semiconductor laser 801 is stabilized as a single mode in the vicinity of 870 nm. Moreover, according to the present example, an HR coating for reflecting light having a wavelength of 870 nm and light having a wavelength of 435 nm (preferably at a 100% reflectance) is provided on the end face 808 . As a result, the fundamental wave and the harmonic wave interact with each other also when being propagated backwards, i.e. from the end face 808 to the semiconductor laser 801 . Therefore, the interaction length obtained by the polarization inversion type bulk element 805 becomes twice as large, so that the conversion efficiency into the harmonic wave becomes four times as large. By so adjusting the temperature of the polarization inversion type bulk element 805 that the phase matching wavelength is tuned to be 870 nm, as in Example 5, the fundamental wave (870 nm) coupled to the polarization inversion type bulk element 805 is converted into a harmonic wave having a wavelength of 435 nm. As a result, blue light goes out through the incident end face 807 of the polarization inversion type bulk element 805 . The blue light is separated from the fundamental wave by the wavelength selection mirror 806 . The polarization inversion type bulk element 805 has a length of 2 mm and a conversion efficiency of 32%/W. As a result, a harmonic wave of about 1.6 mW is obtained. According to the short wavelength light source apparatus of the present example, the reflectance of the end face 808 is made large. As a result, not only that the harmonic light is obtained with higher stability but also the interaction length of the optical wavelength converting element (polarization inversion type waveguide element) 805 is increased. Therefore, the present example has a considerable practical significance. The wavelength selection mirror 806 inserted in the optical system composed essentially of the collimating lens 802 and the focusing lens 804 may alternatively be formed on the thin film optical element 803 . In that case, the number of the component elements of the short wavelength light source apparatus would be reduced, thereby facilitating further miniaturization of the short wavelength light source apparatus. An even higher harmonic output power can be obtained by using a 1 W-class single longitudinal mode laser including a light amplifying portion as the pump laser source. Moreover, the harmonic output power can be stabilized by providing a rotation mechanism and a feedback circuit for the thin film optical element 803 , as in Example 3. Example 12 Although the polarization inversion type bulk element 805 including a substrate composed of LiTaO 3 (LT) is used as an optical wavelength converting element in the short wavelength light source apparatus shown in Figure 8 , similarly stable short wavelength light can be obtained by using: a polarization inversion type bulk element including a substrate composed of LiNbO 3 (LN) or KTiOPO 4 (KTP); or an organic or inorganic non-linear optical crystal (a ring resonator structure, phase matching type waveguide device, etc.) having a large non-linear optical constant, e.g. KNbO 3 (KN). In this case, too, it is preferable to provide an AR coating which reflects neither light having a wavelength of 870 nm nor light having a wavelength of 435 nm on an incident end face of the optical wavelength converting element, and to provide an HR coating for reflecting light having a wavelength of 870 nm and light having a wavelength of 435 nm on an outgoing end face of the optical wavelength converting element. Example 13 It is applicable to provide a transmission type filter composed essentially of a dielectric multilayer film 903 on an outgoing end of the semiconductor laser 901 , as is shown in Figure 9A . It is also applicable to provide a transmission type filter or reflective type filter composed essentially of a dielectric multilayer film 904 on an incident end face of a polarization inversion type waveguide element 902 . In this case, too, the oscillation wavelength of the semiconductor laser 901 can be stabilized. Thus, stable wavelength conversion and harmonic light can be provided. Hereinafter, a short wavelength light source apparatus in which a transmission type filter A03 is provided on an incident end face of the polarization inversion type waveguide element 902 will be described with reference to Figure 10A . Light emitted from the semiconductor laser A01 has an emission angle of about 2° with respect to an outgoing end face of the semiconductor laser A01 . The outgoing end face of the semiconductor laser A01 is disposed at a distance of about 10 µm from an incident end face of a polarization inversion type waveguide element A02 (polarization inversion type waveguide element A02 in Figure 9B ). The normal axis of the incident end face of the polarization inversion type waveguide element A02 is disposed at an angle of 8° with the optical axis of the semiconductor laser A01 . A transmission type filter A03 has a transmittance of 80% for light having a wavelength of 870 nm which is incident thereto at an incident angle of 8°. The transmission spectrum of the transmission type filter A03 has a width of 1 nm. Although light having a wavelength in the vicinity of 870 nm is reflected by the transmission type filter A03 , the reflected light is not fed back to an active layer of the semiconductor laser A01 , since the normal axis of the transmission type filter A03 is disposed at an angle of 8° with the optical axis of the semiconductor laser A01 . On the other hand, an optical waveguide A04 formed in the polarization inversion type waveguide element A02 is disposed at angle of 3.6° with the normal axis of the incident end face of the polarization inversion type waveguide element A02 . This is so that the amount of light coupled to the waveguide A04 becomes maximum, since light entering a material having a refractive index (n) of 2.2 at an incident angle of  travels through the material at an angle of  1 , the angles  and  1 satisfying the following relationship:  1 = sin -1 (sin  / n) Thus, light coupled to the waveguide A04 is reflected by an outgoing end face of the polarization inversion type waveguide element A02 , led back through the transmission type filter A03 , and fed back to the active layer of the semiconductor laser A01 , thereby stabilizing the oscillation wavelength of the semiconductor laser A01 at 870 nm. An HR coating for reflecting light having a wavelength of 870 nm and an AR coating which does not reflect light having a wavelength of 435 nm is provided on the outgoing end face of the polarization inversion type waveguide element A02 . The light coupled to the waveguide A04 is converted into light having a wavelength of 435 nm, which goes out through the outgoing end face of the polarization inversion type waveguide element A02 . Although no optical system is provided between the semiconductor laser A01 and the polarization inversion type waveguide element A02 in the short wavelength light source apparatus shown in Figure 10A , it is applicable to provide one. When providing a reflective type filter instead of the transmission type filter A03 , it is preferable to dispose the optical axis of the semiconductor laser A01 perpendicularly with respect to the incident end face of the polarization inversion type waveguide element A02 so as to increase the efficiency with which light is coupled to the waveguide A04 . Although it is commonly practiced to fabricate polarization-inverted layers and a waveguide in a z-plate LiTaO 3 , the polarization inversion type waveguide element A02 shown in Figure 10A is formed of an x-plate LiTaO 3 , since such a polarization inversion type waveguide element is more preferable because of the large coupling coefficient thereof with the semiconductor laser A01 . The coupling efficiency between the semiconductor laser A01 and the polarization inversion type waveguide element A02 can be still more increased by further providing a λ/2 plate, which is commonly used for improving the coupling efficiency between a z-plate polarization inversion type waveguide element and a semiconductor laser, on the incident end face of the LiTaO 3 substrate. By thus miniaturizing the light source, the entire device incorporating the light source can be miniaturized, and temperature control is also facilitated, thereby making the device more practical. Similar effects can be obtained by providing a reflective type filter on the outgoing end face of the polarization inversion type waveguide element A02 . Example 14 In the short wavelength light source apparatus of Example 13, the polarization inversion type waveguide element A02 is used as an optical wavelength converting element. However, similarly stable short wavelength light can be obtained by using a polarization inversion type bulk element including a substrate composed of LiTaO 3 (LT), LiNbO 3 (LN) or KTiOPO 4 (KTP) or a non-linear optical crystal (a ring resonator structure, phase matching type waveguide device, etc.) having a large non-linear optical constant, e.g. KNbO 3 (KN), as an optical wavelength converting element, and by providing a thin film optical element on the incident end face or the outgoing end face of the optical wavelength converting element. Hereinafter, a short wavelength light source apparatus incorporating a polarization inversion type bulk element A06 will be described with reference to Figure 10B . As is shown in Figure 10B , light emitted from a semiconductor laser A05 is incident to an incident end face of the polarization inversion type bulk element A06 formed in a LiTaO 3 substrate. A transmission type filter A07 provided on the incident end face of the polarization inversion type bulk element A06 transmits only light having a wavelength in the vicinity of 870 nm, which light is led to an outgoing end face of the polarization inversion type bulk element A06 . The outgoing end face of the polarization inversion type bulk element A06 is processed to a spherical shape, and has a curvature of 10 mm. An HR coating for reflecting light having a wavelength of 870 nm and an AR coating which does not reflect light having a wavelength of 435 nm are provided on the outgoing end face of the polarization inversion type bulk element A06 . Light reflected by the outgoing end face of the polarization inversion type waveguide element A06 is transmitted back through the transmission type filter A07 , and is focused on an outgoing end face of the semiconductor laser A05 so as to be optically fed back to an active layer thereof. As a result, the oscillation wavelength of the semiconductor laser short wavelength light source apparatus is stabilized in the vicinity of 870 nm. Thus, light having a wavelength of 435 nm, obtained through a wavelength conversion by the polarization inversion type bulk element A06 goes out through the outgoing end face of the polarization inversion type bulk element A06 . The incident end face of the polarization inversion type bulk element A06 has been cut out at an angle of about 8°, so that light reflected by the transmission type filter A07 is not fed back to the semiconductor laser A05 . Similar effects can be obtained by further providing a reflective type filter on the outgoing end face of the polarization inversion type bulk element A06 . Example 15 According to a short wavelength light source apparatus of the present invention, the oscillation wavelength of a semiconductor laser can be varied by varying an angle  at which a thin film optical element is disposed. By so configurating a polarization inversion type bulk element or waveguide element of the short wavelength light source apparatus as to have a polarization inversion period of a chirp structure or a split structure, light having short wavelengths corresponding to the varying oscillation wavelengths of the semiconductor laser can be obtained. In other words, the wavelength of the short wavelength light can be tuned by varying the incident angle of laser light at which the laser light enters the thin film optical element. Figure 11A shows the polarization inversion type waveguide element 106 shown in Figure 1 . In the polarization inversion type waveguide element 106 , the first order polarization-inverted layers 107 having a period of 4.0 µm are uniformly formed in the LiTaO 3 substrate 109 so that quasi-phase matching is achieved for light having a wavelength of 870 nm. Figure 11B shows a polarization inversion type waveguide element having a split structure. The polarization inversion type waveguide element shown in Figure 11B is 15 mm long. The polarization inversion type waveguide element is divided into three regions I , II , and III . Each region is 5 mm long. The polarization-inverted layers in region I have a period of 3.6 µm; the polarization-inverted layers in region II have a period of 3.8 µm; and the polarization-inverted layers in region III have a period of 4.0 µm. Quasi-phase matching is achieved for light having wavelengths of 850 nm, 860 nm, and 870 nm in regions I , II , and III , respectively. Regions I , II , and III output harmonics having wavelengths of, respectively, 425 nm, 430 nm, and 435 nm. Figure 12 shows a short wavelength light source apparatus incorporating a polarization inversion type waveguide element C06 which is identical with the polarization inversion type waveguide element shown in Figure 11B . Reference numeral C01 denotes a 150 mW-class single longitudinal mode semiconductor laser; C02 denotes a collimating lens (N.A.: 0.55); C03 denotes a thin film optical element; C04 denotes a λ/2 plate; and C05 denotes a focusing lens for coupling light to a waveguide formed in the polarization inversion type waveguide element. The thin film optical element C03 is a band-pass filter composed essentially of a dielectric multilayer film having a transmittance of about 85%, and has a transmission spectrum with peak wavelengths of 870 nm, 860 nm, and 850 nm at incident angles of 1°, 15°, and 20°, respectively. The wavelength of the output harmonic wave can be varied by adjusting the angle  at which the thin film optical element C03 shown in Figure 12 is disposed. The polarization inversion type waveguide element C06 includes an optical waveguide (width: 4 µm) and the polarization-inverted layers shown in Figure 11B formed in a LiTaO 3 substrate. An HR coating for reflecting light having a wavelength in the range of 850 nm to 870 nm is provided on an outgoing end face C08 of the polarization inversion type waveguide element C06 . An AR coating which does not reflect light having a wavelength in the range of 850 nm to 870 nm is provided on an incident end face C07 of the polarization inversion type waveguide element C06 . Light emitted from the semiconductor laser C01 is focused by the focusing lens C05 on the incident end face C07 of the polarization inversion type waveguide element C06 so as to be coupled to the waveguide. An outgoing end face of the semiconductor laser C01 and the incident end face C07 of the polarization inversion type element C06 are confocally disposed. The outgoing end face of the semiconductor laser C01 also constitutes a confocal system with the outgoing end face C08 of the polarization inversion type waveguide element C06 . As a result, light reflected by the outgoing end face C08 is all optically fed back to an active layer of the semiconductor laser C01 . If the thin film optical element C03 is not provided, the above-mentioned reflected light (returned light) may cause the semiconductor laser C01 to have mode hopping or to have a multitude of modes. By disposing the thin film optical element C03 between the semiconductor laser C01 and the polarization inversion type waveguide element C06 at a certain angle with the optical axis, it is ensured that only light having a corresponding wavelength in a narrow range of band from 850 nm to 870 nm, emitted from the semiconductor laser C01 , is incident to the incident end face C07 of the polarization inversion type waveguide element C06 , so that only light having that wavelength which is reflected by the outgoing end face C08 is optically fed back to the semiconductor laser C01 . As a result, the semiconductor laser C01 stably oscillates in a single longitudinal mode, the oscillation wavelength being stabilized. By rotating the thin film optical element C03 so that the oscillation wavelength of the semiconductor laser C01 coincides with the phase matching wavelengths of the respective split regions I , II , and III , the oscillation wavelength can be made to coincide with the respective phase matching wavelengths of 850 nm, 860 nm, and 870 nm. As a result, the laser light is converted into light having wavelengths of 425 nm, 430 nm, or 435 nm, which is obtained through the outgoing end face C08 of the polarization inversion type waveguide element C06 . As a result, a harmonic wave of about 2.5 mW is obtained at each wavelength, the fundamental wave power of about 70 mW being coupled to the waveguide. An AR coating for reflecting harmonics is provided on the outgoing end face C08 of the polarization inversion type waveguide element C06 . As a rotation mechanism for rotating the thin film optical element C03 , a rotation monitor and/or a piezo element may be used. The output power of the short wavelength light source apparatus can be stabilized by monitoring the harmonic output power and feeding back the monitored harmonic output power to the rotation mechanism. Figure 11C shows a polarization inversion type waveguide element having a chirp structure. The polarization inversion type waveguide element is 15 mm long. The polarization-inverted layers have a period of 3.6 µm at an end face 01 , and a period of 4.0 µm at an end face B02 , thus providing a linear chirp structure. This polarization inversion type waveguide element can be substituted for the polarization inversion type waveguide element C06 in the polarization inversion type waveguide element C06 shown in Figure 12 . In that case, the wavelength of the output harmonic wave can be continually varied by varying the angle  at which the thin film optical element C03 is disposed from 0° to 20°. Although light reflected by the outgoing end face C08 of the polarization inversion type waveguide element C06 is optically fed back to the semiconductor laser C01 in the short wavelength light source apparatus shown in Figure 12 , a wavelength-variable short wavelength light source apparatus can also be realized by optically feeding back light reflected by the incident end face C07 , as is shown in Figure 3 . A short wavelength light source apparatus having four times as large an harmonic output power as that of the short wavelength light source apparatus shown in Figure 12 can be realized by providing an HR coating for reflecting the fundamental wave and the harmonic wave and by extracting the harmonic wave by means of a wavelength selection mirror. The wavelength-variable short wavelength light source apparatus according to the present example, capable of generating light in the blue wavelength band, has much use in the fields of measurement and communication as well as a wide range of applicability. Example 16 Although the polarization inversion type waveguide element C06 is used as an optical wavelength converting element in the short wavelength light source apparatus shown in Figure 12 , a similar wavelength-variable short wavelength light source apparatus can be realized by using a polarization inversion type bulk element having a split structure or a chirp structure. It is also applicable to use a LiNbO 3 or KTP substrate instead of a LiTaO 3 substrate. Example 17 Although a transmission type filter is used as a thin film optical element in the short wavelength light source apparatus shown in Figure 12 , a similar wavelength-variable short wavelength light source apparatus can be realized by using a reflective type filter. Example 18 The phase matching wavelength of a polarization inversion type waveguide element can be adjusted by varying the refractive index of the substrate of the polarization inversion type waveguide element through temperature control or application of an electric field. The wavelength of the output harmonic light can be varied by adjusting the phase matching wavelength of the polarization inversion type waveguide element. A short wavelength light source apparatus according to the present example is identical with that shown in Figure 12 except for the configuration of the optical wavelength converting element. The optical wavelength converting element of the present example is a polarization inversion type waveguide element including an optical waveguide (width: 4 µm) and polarization-inverted layers (period: 3.8 µm) formed in a LiTaO 3 substrate. A thin film resistor is formed on the optical waveguide. By varying the temperature of the polarization inversion type waveguide element with the use of the thin film resistor, the phase matching wavelength can be adjusted to be any value in the range of 850 nm to 870 nm. An HR coating for reflecting light having a wavelength in the range of 850 nm to 870 nm, and an AR coating which does not reflect harmonic waves are provided on an outgoing end face of the polarization inversion type waveguide element. An AR coating which does not reflect light having a wavelength in the range of 850 nm to 870 nm is provided on an incident end face of the polarization inversion type waveguide element. By adjusting the phase matching wavelength to be a desired value in the range of 850 nm to 870 nm and by tuning the oscillation wavelength of the semiconductor laser to be the phase matching wavelength, a harmonic light having a desired wavelength in the range of 425 nm to 435 nm is obtained through the outgoing end face of the polarization inversion type waveguide element. As a result, a harmonic wave of about 10 mW is obtained, laser light of about 70 mW being coupled to the waveguide. In cases where a thin film electrode is formed on the waveguide, instead of the thin film resistor, the phase matching wavelength of the polarization inversion type waveguide element can be varied by varying the applied voltage. The wavelength-variable short wavelength light source apparatus according to the present example, capable of generating light in the blue wavelength band, has much use in the fields of measurement and communication as well as a wide range of applicability. Example 19 The oscillation wavelength of a semiconductor laser of a short wavelength light source apparatus according to the present invention can also be adjusted to be the phase matching wavelength of the polarization inversion type waveguide element by using a Bragg's reflective type thin film optical element composed essentially of a plurality of periodically formed layers. A method for producing a Bragg's reflective type thin film optical element will be described. SiO 2 (refractive index: 1.46), SiO 2 having a different composition (refractive index: 1.48) are alternately formed with a period of 0.27 µm on a quartz substrate by means of an EB vapor deposition apparatus. About 100 such layers are laminated to form the Bragg's reflective type thin film optical element. A Bragg's reflective type thin film optical element is very practical because the spectrum width thereof can be easily adjusted by varying the number of the layers laminated. The oscillation wavelength of a semiconductor laser of a short wavelength light source apparatus incorporating this Bragg's reflective type thin film optical element instead of the band-stop filter shown in Figure 1 is stabilized at the phase matching wavelength. A stable harmonic output power can be obtained without any mode hopping. In the above-mentioned example, the Bragg's reflective type thin film optical element is formed on a quartz substrate so as to form a separate optical element. However, similar effects can be attained by forming a Bragg's reflective type thin film optical element directly on an incident end face or an outgoing end face of the polarization inversion type waveguide element, as shown in Figure 10 , or on an outgoing end face of the semiconductor laser, thus providing a stable blue/green light source apparatus. A Bragg's reflective type filter can also be realized by a holographical method where laser light of a predetermined wavelength is incident to a photorefractive material such as LT and LN doped with Fe in two directions so that the two laser light beams interfere with each other. Example 20 Each of the transmission type or reflective type filters shown in Figures 1 , 3 , 5 , 6 , 7 , 8 , 12 , and 13 is a thin film optical element obtained by forming a dielectric multilayer film on a glass substrate having a thickness of 0.5 mm. It was found out that the oscillation wavelength becomes especially stable when the glass substrate has a thickness in the range of 0.2 mm to 2.0 mm. The oscillation wavelength becomes unstable when the thickness of the substrate is 0.2 mm or less because of the warping of the substrate after polishing. The oscillation wavelength has an aberration when the thickness of the substrate is 2.0 mm or more owing to the thickness of the substrate. Example 21 In the examples shown in Figures 1 , 3 , 5 , 6 , 7 , 8 , 12 , and 13 , a thin film optical element including a dielectric multilayer film formed on a glass substrate is used as an optical element for transmitting or reflecting light in a predetermined wavelength band. The longitudinal mode of the semiconductor laser can also be locked and stabilized by using the following as an optical element: an etalon element including a highly planar element polished so as to be highly planar and so as to have a high degree of parallelism and a reflective film formed on each side of the highly planar element; or a birefringence filter that utilizes interference between normal light and extraordinary light in a birefringence crystal plate. Hereinafter, an etalon element will be described. Etalon composed of synthetic quartz having a thickness of 50 µm, a reflectance of 85%, and a parallelism of λ/10 is used in the present example. Etalon has peak transmittances at certain intervals when transmittance is regarded as a function of the wavelength of incident light. The interval Δv between modes is derived from the following equation: Δv=λ 2 / (2nL) wherein n represents the refractive index of the material, and L represents the thickness of the material. In the present example, Δv is 5 nm. Next, finesse F of the etalon is derived. The finesse F is expressed as a ratio between Δv and the half width Δv 1/2 of a certain mode. The finesse F is determined by such factors as reflectance (reflection finesse), degree of planarity and degree of parallelism (planarity finesse), and coarseness of the plane, as is expressed by the following equation: 1 / F 2 = (1 / F r 2 ) + (1 / F d 2 ) wherein F r 2 represents the reflection finesse, and F d 2 represents the planarity finesse. The reflection finesse F r 2 and the planarity finesse F d 2 are expressed by the following equations: F r 2 = π 2 R/ (1 - R) 2 F d = S / 2 wherein R represents the reflectance, and S is a value obtained by dividing λ by the degree of parallelism of the etalon. The finesse F of the etalon of the present example is 5. The half width Δv 1/2 of the etalon of the present example is derived to be 1 nm from the following equation: Δv 1/2 = Δv / F The value 1 nm of the half width of the etalon of the present example is substantially the same as that of a transmission type filter (thin film optical element). By disposing this etalon between a semiconductor laser and an optical wavelength converting element, the longitudinal mode of the semiconductor laser is stabilized as a single mode. Moreover, the oscillation wavelength of the semiconductor laser becomes stable by increasing the reflectance and the degree of planarity of the etalon so that the half width Δv 1/2 further reduces. Next, a birefringence filter will be described. An interference pattern can be obtained by radiating light on a crystal panel interposed between two polarizing plates, corresponding to the change in the wavelength of the incident light. By laminating several layers of such composites each composed of a crystal plate interposed between polarizing plates (so that the thickness of the multilayer varies from d to 2d, 4d, .., etc.), the interference patterns overlap with one another so that a sharp peak is left. Thus, a band-pass filter having a transmission spectrum width of about 0.1 nm is realized. The oscillation wavelength of the semiconductor laser can be stabilized also by using such a birefringence filter in the place of a thin film optical element. As has been described, according to the present invention, a compact, stable, and high-power short wavelength light source apparatus for generating green or blue light is provided which incorporates as a pump light source a light generating device including an optical element for transmitting or reflecting light in a predetermined wavelength band, and a polarization inversion type waveguide element, a polarization inversion type bulk element, and/or a non-linear optical crystal for converting light emitted from the semiconductor laser into light having a different wavelength. A short wavelength light source apparatus according to the present invention is also capable of providing near infrared light by incorporating a laser medium, and providing stable short wavelength light by adopting an intracavity configuration. Thus, the present invention has a large practical significance because it can provide a short wavelength light source apparatus to be used for an optical disk or for measurement purposes, where the short wavelength light source apparatus is required to have a stable, low-noise output power. Generally, a short wavelength light source apparatus incorporating a semiconductor laser is likely to have the problem of unstable oscillation by the semiconductor laser due to light returning from an end face of an optical wavelength converting element or an end face of a laser medium. However, by using the above-mentioned optical element, light returning (reflected) from such end faces can be utilized for stabilizing the oscillation wavelength of the semiconductor laser, which makes a short wavelength light source apparatus of the present invention very practical. Moreover, the optical component elements can be linearly disposed, thereby facilitating miniaturization of the short wavelength light source apparatus. A peak wavelength of the transmission spectrum of a transmission type filter (thin film optical element) generally has angle dependence. When the incident light enters at an angle of 20° with the optical axis, the peak wavelength of the transmission spectrum shifts by 1.5 nm/deg. When the incident angle is in the vicinity of 10°, the shift amount is 0.9 nm/deg. When the incident angle is 0°, the peak wavelength makes substantially no shift depending on the angle. Taking advantage of the angle dependence of the filter, the oscillation wavelength of the semiconductor laser can be tuned to be the phase matching wavelength of the optical wavelength converting element. On the other hand, the shift amount of the transmission spectrum of a transmission filter is not so large as compared with that of a grating, e.g. 28 nm/deg. Therefore, using a transmission type filter to stabilize the oscillation wavelength of the semiconductor laser makes the necessary optical adjustment easier than using grating feedback technique. It will be appreciated that use of a transmission type filter (thin film optical element) for stabilization of the oscillation wavelength of the semiconductor laser has great practical effects. Moreover, a short wavelength light source apparatus according to the present invention is stable against temperature changes, having a wavelength shift of 0.005 nm/°C, and is also stable against changes in moisture. Since it is not composed of resin unlike a grating, the entire short wavelength light source apparatus is made reliable. Since thin film optical elements are produced by vapor deposition, a mass-production effect is provided, which increases the practicality of the short wavelength light source apparatus. Even more stable oscillation wavelength (oscillation wavelength locking) can be achieved by incorporating an etalon or a birefringence filter, since the half width of the transmission spectrum of such elements can be reduced to about 0.1 nm. By providing a rotation mechanism such as a piezo element or a motor (actuator) on the filter and by conducting a feedback so as to stabilize the output power of the short wavelength light, it becomes possible to obtain an output power which is stable against changes in the refractive index due to temperature changes of the optical wavelength converting element and/or optical damage, i.e. changes in the phase matching wavelength, thereby making the short wavelength light source apparatus very practical. Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.

Claims: A short wavelength light source apparatus comprising: a semiconductor laser (101) including an active layer for emitting laser light; an optical wavelength converting element (106) for receiving at least a portion of the laser light emitted from the semiconductor laser (101) and for generating short wavelength light having a wavelength shorter than the wavelength of the laser light; and an optical element (103) disposed between the semiconductor laser (101) and the optical wavelength converting element (106) for selectively reflecting a portion of the laser light that belongs to a predetermined wavelength band and for feeding back the reflected light to the active layer of the semiconductor laser (101) so as to achieve stable oscillation in a single longitudinal mode, characterized in that the optical element (103) is a thin film optical element (103) comprising dielectric layers, the optical wavelength converting element (106) is a quasi-phase-matching (QPM) type polarization inversion element having a waveguide, and the polarization direction of the laser light emitted from the semiconductor laser (101) is set to coincide with the polarization direction of said waveguide of the optical wavelength converting element (106). A short wavelength light source apparatus according to claim 1, wherein the optical wavelength converting element (106) is formed in a LiNb x Ta 1-x O 3 (0 ≤ x ≤ 1) substrate or a K 1-x Rb x TiOM0 4 (0 ≤ x ≤ 1; M = P or As ) substrate. A short wavelength light source apparatus according to one of claims 1 or 2, wherein the optical element (103) is a Bragg's reflective type thin film optical element comprising dielectric layers having different refractive indices, the dielectric layers being formed with a predetermined period. A short wavelength light source apparatus according to one of claims 1 or 2, wherein the optical element is a thin film optical element (103) including a substrate and a dielectric multilayer film formed on the substrate. A short wavelength light source apparatus according to one of claims 1 or 2, wherein the optical element (103) comprises a dielectric multilayer film (303) formed on an incident end face or an outgoing end face of at least one of the semiconductor laser (101) and the optical wavelength converting element (106). A short wavelength light source apparatus according to one of claims 1 to 5 further comprising more than one optical element beside the thin film optical element (103). A short wavelength light source apparatus according to one of the preceding claims, wherein the optical element (103) includes a thin film (304) functioning as a λ/2 plate for the wavelength of the laser light. A short wavelength light source apparatus according to one of the preceding claims, wherein a λ/2 plate (104) is located between the semiconductor laser (101) and the QPM type polarization inversion element (106). A short wavelength light source apparatus comprising: a semiconductor laser (301) including an active layer for emitting laser light; an optical wavelength converting element (306) for receiving at least a portion of the laser light emitted from the semiconductor laser (301) and for generating short wavelength light having a wavelength shorter than the wavelength of the laser light, wherein the optical wavelength converting element (306) has an incident end face (307) for receiving at least a portion of the laser light and an outgoing end face through which the short wavelength light goes out; and an optical element (303) disposed between the semiconductor laser (301) and the optical wavelength converting element (306); characterized in that the optical element (303) is a thin film optical element (303) comprising dielectric layers for selectively transmitting a portion of the laser light that belongs to a predetermined wave length band; that the optical wavelength converting element is a quasi-phase-matching (QPM) type polarization inversion element having a waveguide; the semiconductor laser (301) and the optical wave length converting element (306) being disposed in such a manner that the portion of the laser light transmitted through the optical element (303) is reflected by the incident end face (307) of the optical wavelength converting element (306) so as to be transmitted back through the thin film optical element (303) and fed back to the active layer of the semiconductor laser (301) so as to achieve stable oscillation in a single longitudinal mode, and that the polarization direction of the laser light emitted from the semiconductor laser (301) is set to coincide with the polarization direction of said waveguide of the optical wavelength converting element (306). A short wavelength light source apparatus according to claim 9, wherein the optical wavelength converting element is formed in a LiNb x Ta 1-x O 3 (0 ≤ x ≤ 1) substrate or a K 1-x Rb x TiOM0 4 (0 ≤ x ≤ 1; M = P or As ) substrate. A short wavelength light source apparatus according to claim 9, wherein the optical wavelength converting element is divided into a plurality of regions having different polarization inversion periods from one another. A short wavelength light source apparatus according to claim 11, wherein the wavelength of light incident to the optical wavelength converting element is adjusted to be a phase matching wavelength of the optical wavelength converting element by varying the angle of the optical element, whereby the wavelength of the short wavelength light is varied. A short wavelength light source apparatus according to claim 9, wherein the optical wavelength converting element has a chirp structure having a varying polarization inversion period. A short wavelength light source apparatus according to claim 13, wherein the wavelength of light incident to the optical wavelength converting element is adjusted to be a phase matching wavelength of the optical wavelength converting element by varying the angle of the optical element, whereby the wavelength of the short wavelength light is varied. A short wavelength light source apparatus according to one of claims 9 to 14, wherein the optical element (503) is a thin film optical element including a substrate and a dielectric multilayer film formed on the substrate. A short wavelength light source apparatus according to one of claims 9 to 14, wherein the optical element comprises a dielectric multilayer film formed on an incident end face or an outgoing end face of at least one of the semiconductor laser and the optical wavelength converting element. A short wavelength light source apparatus according to one of claims 9 to 14, wherein the optical element is disposed at an angle with an optical axis of the semiconductor laser. A short wavelength light source apparatus according to one of claims 9 to 15 further comprising a rotation mechanism for rotating the optical element (503). A short wavelength light source apparatus according to claim 18 further comprising means for varying a phase matching wavelength of the optical wavelength converting element (506), wherein a wavelength of light incident to the optical wavelength converting element (506) is adjusted to be the phase matching wavelength by varying the angle of the optical element, whereby the wavelength of the short wavelength light is varied. A short wavelength light source apparatus according to claim 18, wherein the wavelength of light incident to the optical wavelength converting element (506) is adjusted to be a phase matching wavelength of the optical wavelength converting element (506) by varying the angle of the optical element (503), whereby the wavelength of the short wavelength light is varied. A short wavelength light source apparatus according to claim 18, wherein the rotation mechanism includes a feedback circuit for controlling the output power of the short wavelength light to be constant. A short wavelength light source apparatus according to claim 18, wherein the rotation mechanism includes a piezo element. A short wavelength light source apparatus according to claim 18, wherein the rotation mechanism includes an actuator. A short wavelength light source apparatus according to one of claims 9 to 23 further comprising more than one optical element besides the optical element. A short wavelength light source apparatus according to claim 18, wherein the rotation mechanism includes a thin film functioning as a λ/2 plate for the wavelength of the laser light. A short wavelength light source apparatus according to claims 9 to 24, wherein a λ/2 plate (304) is located between the semiconductor laser (301) and the element (306). A short wavelength light source apparatus comprising: a semiconductor laser (501) including an active layer for emitting laser light; and an optical wavelength converting element (506) for receiving at least a portion of the laser light emitted from the semiconductor laser (501) and for generating short wavelength light having a wavelength shorter than a wavelength of the laser light, wherein the optical wavelength converting element (506) has an incident end face (507) for receiving at least a portion of the laser light and an outgoing end face (508) through which the short wavelength light goes out; and an optical element (503) disposed between the semiconductor laser (501) and the optical wavelength converting element (506), characterized in that the optical wavelength converting element (506) is a quasi-phase-matching (QPM) type polarization inversion element having a waveguide, the optical element (503) is a thin film optical element comprising dielectric layers for selectively transmitting a portion of the laser light that belongs to a predetermined wavelength band, the semiconductor laser (501) and the optical wavelength converting element (506) being disposed in such a manner that the portion of the laser light transmitted through the thin film optical element (503) enters the optical wavelength converting element (506) at the incident end face (507) thereof and thereafter is reflected by the outgoing end face (508) of the optical wavelength converting element (506) so as to be transmitted back through the thin film optical element (503) and fed back to the active layer of the semiconductor laser (501) so as to achieve stable oscillation in a single longitudinal mode, and the polarization direction of the laser light emitted from the semiconductor laser (501) is set to coincide with the polarization direction of said waveguide of the optical wavelength converting element (506). A short wavelength light source apparatus according to claim 27, wherein the optical wavelength converting element (506) is formed in a LiNb x Ta 1-x O 3 (0 ≤ x ≤ 1) substrate or a K 1-x Rb x TiOM0 4 (0 ≤ x ≤ 1; M = P or As ) substrate. A short wavelength light source apparatus according to claim 27, wherein the optical wavelength converting element (506) is divided into a plurality of regions having different polarization inversion period from one another. A short wavelength light source apparatus according to claim 29, wherein the wavelength of light incident to the optical wavelength converting element (506) is adjusted to be a phase matching wavelength by varying the angle of the optical element, whereby the wavelength of the short wavelength light is varied. A short wavelength light source apparatus according to claim 27, wherein the optical wavelength converting element (506) has a chirp structure having a varying polarization inversion period. A short wavelength light source apparatus according to claim 31, wherein the wavelength of light incident to the optical wavelength converting element is adjusted to be a phase matching wavelength of the optical wavelength converting element by varying the angle of the optical element, whereby the wavelength of the short wavelength light is varied. A short wavelength light source apparatus according to one of claims 27 to 32, wherein the optical element is a thin film optical element (503) including a substrate and a dielectric multilayer film formed on the substrate. A short wavelength light source apparatus according to one of claims 27 to 32, wherein the optical element (503) comprises a dielectric multilayer film formed on an incident end face or an outgoing end face of at least one of the semiconductor laser (501) and the optical wavelength converting element (506). A short wavelength light source apparatus according to one of claims 27 to 32, wherein the optical element (503) is disposed at an angle with the optical axis of the semiconductor laser (501). A short wavelength light source apparatus according to one of claims 27 to 35 further comprising a rotation mechanism for rotating the optical element (503). A short wavelength light source apparatus according to claim 36 further comprising means for varying a phase matching wavelength of the optical wavelength converting element, wherein the wavelength of light incident to the optical wavelength converting element is adjusted to be the phase matching wavelength by varying the angle of the optical element, whereby the wavelength of the short wavelength light is varied. A short wavelength light source apparatus according to claim 36, wherein the wavelength of light incident to the optical wavelength converting element is adjusted to be a phase matching wavelength of the optical wavelength converting element by varying the angle of the optical element, whereby the wavelength of the short wavelength light is varied. A short wavelength light source apparatus according to claim 36, wherein the rotation mechanism includes a feedback circuit for controlling the output power of the short wavelength light to be constant. A short wavelength light source apparatus according to claim 36, wherein the rotation mechanism includes a piezo element. A short wavelength light source apparatus according to claim 36, wherein the rotation mechanism includes an actuator. A short wavelength light source apparatus according to one of claims 27 to 41, further comprising more than one optical element beside the thin film optical element. A short wavelength light source apparatus according to one of claims 27 to 42, wherein the thin film optical element (603) includes a thin film (604) functioning as a λ/2 plate for the wavelength of the laser light. A short wavelength light source apparatus according to one of claims 27 to 42, wherein a λ/2 plate (504) is located between the semiconductor laser (501) and the element. A short wavelength light source apparatus comprising: a semiconductor laser (601) including an active layer for emitting laserlight; an optical wavelength converting element (606) for receiving at least a portion of the laser light emitted from the semiconductor laser (601) and for generating short wavelength light having a wavelength shorter than the wavelength of the laser light, wherein the optical wavelength converting element (606) includes a waveguide and has an incident end face (680) for receiving at least a portion of the laser light and an outgoing end face (608), and an optical element (603) disposed between the semiconductor laser (601) and the optical wavelength converting element (605), characterized in that the optical wavelength converting element is a quasi-phase-matching (QPM) type polarization inversion element, the optical element is a thin film optical element (603) comprising dielectric layers for selectively transmitting a portion of the laser light that belongs to the predetermined wavelength band, a wavelength selection mirror (607) is provided for selectively transmitting a portion of the laser light that belongs to a predetermined wavelength band and for reflecting the short wavelength light, the wavelength selection mirror (606) being disposed between the semiconductor laser (601) and the optical wavelength converting element (606), and a portion of the laser light that belongs to the predetermined wavelength band is transmitted through the thin film optical element (603) and the wavelength selection minor (606), reflected by the incident end face (680) of the optical wavelength converting element (606), and thereafter is transmitted back through the thin film optical element (603) and the wavelength selection mirror (606) so as to be fed back to the active layer of the semiconductor laser (601) in order to achieve stable oscillation in a single longitudinal mode, the short wavelength light generated by the optical wavelength converting element (606) is reflected by the outgoing end face (680) of the optical wavelength converting element (606), propagated back through the waveguide, and thereafter is reflected in a predetermined direction by the wavelength selection mirror (607), and the polarization direction of the laser light emitted from the semiconductor laser (601) is set to coincide with the polarization direction of a waveguide of the optical wavelength converting element (605). A short wavelength light source apparatus according to claim 45, wherein the optical wavelength converting element (605) is formed in a LiNb x Ta 1-x O 3 (0 ≤ x ≤ 1) substrate or a K 1-x Rb x TiOM0 4 (0 ≤ x ≤ 1; M = P or As ) substrate. A short wavelength light source apparatus according to one of claims 45 or 46, wherein the optical wavelength converting element (605) is divided into a plurality of regions having different polarization inversion period from one another. A short wavelength light source apparatus according to claim 47, wherein the wavelength of light incident to the optical wavelength converting element (605) is adjusted to be a phase matching wavelength of the optical wavelength converting element by varying the angle of the optical element, whereby the wavelength of the short wavelength light is varied. A short wavelength light source apparatus according to claim 46, wherein the optical wavelength converting element has a chirp structure having a varying polarization inversion period. A short wavelength light source apparatus according to claim 49, wherein the wavelength of light incident to the optical wavelength converting element is adjusted to be a phase matching wavelength of the optical wavelength converting element by varying the angle of the optical element, whereby the wavelength of the short wavelength light is varied. A short wavelength light source apparatus according to one of claims 45 to 50, wherein the optical element (603) is a thin film optical element including a substrate and a dielectric multilayer film formed on the substrate. A short wavelength light source apparatus according to one of claims 45 to 50, wherein the optical element comprises a dielectric multilayer film formed on an incident end face or an outgoing end face of at least one of the semiconductor laser (601) and the optical wavelength converting element (605). A short wavelength light source apparatus according to one of claims 45 to 50, wherein the optical element (603) is disposed at an angle with the optical axis of the semiconductor laser (801). A short wavelength light source apparatus according to one of claims 45 to 53 further comprising a rotation mechanism for rotating the optical element (603). A short wavelength light source apparatus according to claim 54 further comprising means for varying the phase matching wavelength of the optical wavelength converting element (605), wherein the wavelength of light incident to the optical wavelength converting element is adjusted to be the phase matching wavelength by varying the angle of the optical element (603), whereby the wavelength of the short wavelength light is varied. A short wavelength light source apparatus according to claim 54, wherein the wavelength of light incident to the optical wavelength converting element is adjusted to be a phase matching wavelength of the optical wavelength converting element by varying the angle of the optical element, whereby the wavelength of the short wavelength light is varied. A short wavelength light source apparatus according to claim 54, wherein the rotation mechanism includes a feedback circuit for controlling the output power of the short wavelength light to be constant. A short wavelength light source apparatus according to claim 54, wherein the rotation mechanism includes a piezo element. A short wavelength light source apparatus according to claim 54, wherein the rotation mechanism includes an actuator. A short wavelength light source apparatus according to one of claims 45 to 59 further comprising more than one optical element besides the optical element (603). A short wavelength light source apparatus according to one of claims 45 to 60, wherein the optical element includes a thin film functioning as a λ/2 plate for the wavelength of the laser light. A short wavelength light source apparatus according to one of claims 45 to 61, wherein the wavelength selection mirror functioning as a transmission type film for the laser light and functioning as a reflective type film for the short wavelength light is provided on the optical element. A short wavelength light source apparatus according to one of claims 45 to 62, wherein a λ/2 plate is located between the semiconductor laser (601) and the converting element (605).

Inventors: KATO MAKOTO, KITAOKA YASUO, YAMAMOTO KAZUHISA, KATO, MAKOTO, KITAOKA, YASUO, YAMAMOTO, KAZUHISA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: H01S3
IPC codes: H01S3, G02F1, H01S5